Electronic musical instrument



Jan. 30, 1951 T. J. GEORGE ELECTRONIC MUSICAL INSTRUMENT 2 Sheets-Sheet 1 Filed July 30, 1945 Q mm.) W h A L? k. III. 411 v 11111- l N hm Nb H mm m gxix E il T F INVENTOR.

\N SEQ 5%MM E Jan. 30, 1951 T. J. GEORGE ELECTRONIC MUSICAL INSTRUMENT 2 Sheets-Sheet 2 Filed July ISO, 1945 INVENTOR.

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fatented Jan. 30, 195 1 UNITED STATES PATENT OFFICE 14 Claims.

The present invention relates to electronic circuits, and more particularly to such circuits for use in electronic musical instruments such as organs.

Heretoiore numerous electronic organs have been devised for the generation of musical ire quencies required in such instruments. A num ber of electronic organs have been proposed which used vacuum tube oscillators and locked oscillators for frequency division to obtain the desired signals. Such frequency dividers, however, have certain disadvantages particularly when circuit constants change due to temperature, humidity, and age. Tube characteristics change slightly with time and use, and other circuit components such as resistors and capacitors change with temperature and humidity as well as with age. Such changes produce tones of the wrong musical pitch. In electronic organs of this type it is not uncommon to use oscillators or tone generators numbering from seventy to one hundred which must maintain a certain constant relation therebetween in order to provide the true musical intervals.

In the event that there is an appreciable change in the tube characteristics or circuit constants, the oscillators may get out of synchronism and also the frequency dividers may fail to divide properly. It, therefore, becomes evident that it is highly desirable to provide an electronic musical instrument in which moderate changes in tube characteristics or circuit constants have no appreciable efiect on the character of the signals generated.

In accordance with the present invention it is proposed to provide a plurality of vacuum tube circuits having a high degree of stability where the changes of components due to age, temperature, and humidity and the changes in the tube characteristics have no appreciable effeet. In order to avoid the diificulty heretofore encountered in the arrangements utilizing frequency dividers, it is proposed to utilize frequency doublers employing a relatively simple stable circuit. A plurality of full wave rectifier or transrectifier frequency douglers arranged in cascade relation with interposed amplifiers are provided for each fundamental tone and the octaves thereof.

Certain electronic organs heretofore have used oscillators or tone generators producing a relatively pure fundamental tone which requires a relatively complicated circuit for tone synthesis. In accordance with the present invention it is proposed to supply to a greater extent the natural harmonics necessary for a better quality tone. This may be obtained by arranging the interposed amplifiers between the frequency doublers to generate harmonics by distortion.

In addition it is desirable to provide in electronic organs a chorus or celeste effect. It furthermore is desirable to provide for a vibrato effect. These effects together with a control of a slow and rapid attack may also be obtained in accordance with the present invention.

It, therefore, is an object of the present invention to provide an improved electronic circuit for overcoming certain of the disadvantages above mentioned.

Another object of the present invention is to provide an electronic circuit for musical instruments having frequency doubling circuits provided with means for eliminating the fundamental component.

Still another object of the present invention is to provide an electronic musical instrument having a master oscillator associated with a plurality of cascade frequency doublers.

Still another object of the present invention is to provide electronic circuits for a musical instrument which circuits are relatively independent of changes due to humidity, temperature, and age of the circuit components.

A still further object of the present invention is to provide an improved master oscillator circuit for an electronic musical instrument.

Still another object of the present invention is to provide an improved arrangement for obtaining simultaneously or selectively from each stage of a plurality of cascade amplifiers a keyed signal and a keyed envelope signal.

A still further object of the present invention is to provide an improved circuit arrangement for obtaining in an electronic musical instrument a vibrato eflect.

Another object of the present invention is to provide an improved and simplified circuit for obtaining in an electronic musical instrument a celeste eiiect.

A still further object of the present invention is to provide an improved circuit for producing in an electronic musical instrument the effect of reverberation.

Still another object of the present invention is to provide an improved circuit for the pedal bass notes of an electronic organ.

Other and further objects of the present vention subsequently will become apparent reference to the following description taken in connection with the accompanying drawing wherein anode of the pentode tube Figure 1 is a circuit diagram representing the essentials of the present invention;

Figure 2 is a circuit diagram suitable for the production of a celeste effect;

Figure 3 is a circuit diagram suitable for the production of a vibrato effect;

Figure 4 is a circuit diagram of an arrangement for producing pedal bass notes in an electronic organ; and

Figures 5 and 6 illustrate certain variations or modifications which may be made in portions of the circuit shown in Figure 1.

Referring more particularly to Figure 1 there.

is shown a circuit arrangement employing a master oscillator H which is connected to a coupling stage I2 which supplies energy to a frequency doubler and amplifier unit IS. The circuit utilizes a plurality of similar frequency doubler and amplifier units l3 in sufficient number to provide the necessary octaves for the musical instrument. These amplifiers are terminated by a simultaneous frequency doubling and amplifying stage [4.

The master oscillator H employs a multigrid tube I5 such as a tetrode or a pentode connected as illustrated as a tetrode. This tube is associated with a tuned circuit comprising a tapped inductor l6 and a capacitor ll selected so as to have a high Q value. One end of the inductor is coupled through a capacitor E8 to the grid of the pentode 55 which is connected through a grid resistor 19 to its cathode. The cathode of the pentode vacuum tube is also connected to an intermediate point on the inductor H6. The other end of the inductor I6. is grounded. The remaining two grids in the 'pentode tube 15, or the screen if a tetrode is used, are connected to a source of potential supplied by a conductor 2i. These grids are connected by a capacitor 22 to the grounded end of the inductor It. The two grids which were connected together to a source of plate potential therefore serve as the anode of the oscillator circuit. The E5 is connected through a variable resistor 23 and a fixed resistor 24 to the potential conductor H. The common juncture between the fixed resistor 24 and the variable resistor 23 is connected to a coupling capacitor 25 which is connected to the grid end of the inductor H5. The capacitor 25 serves to complete a reverse feed back circuit, and preferably the capacitor 25 is selected so as to have a value not exceeding one fourth of the value of the capacitor H. The reverse feed back connection has the effect of lowering the resonant frequency of the tuned circuit comprising the inductor l6 and the capacitor IT. The fixed resistor 2!; is selected so as to have a resistance value equal to the impedance value of the capacitor 25. The output of the'circuit is obtained between ground and the cathode of the vacuum tube 15.

The adjustable resistor 23, which may have a value of approximately 500,000 ohms, is used to control the amount of feed back thereby to control the frequency of the oscillator H. By means of the adjustable resistor 23 a frequency change which is approximately ten percent of the value of the frequency determined by the inductor l6 and the capacitor I! may be obtained.

The master oscillator H is connected to the coupling stage l2 employing preferably a tricde vacuum tube 26 which may be provided with a grounded self-bias resistor 21. The anode of the vacuum tube 26 is connected to an anodepotential conductor 28 through the primary winding of a coupling transformer 29. The secondary winding of the coupling transformer 29 is connected to the input terminals of a stage comprising a frequency doubler and amplifier.

The apparatus indicated 10y reference numeral 83 comprises a frequency doubler formed by a full wave transrectifier or rectifier. As shown in Figure 1 this rectifier may be contained within the same envelope of a vacuum tube serv ing as an amplifier for the frequency doubler. Since it is desired to keep at a minimum the number of components and vacuum tubes employed in an electronic instrument such as an organ, it has been found convenient to utilize a double diode pentode vacuum tube 3! to serve as the rectifier tube for the frequency doubler and as the amplifier for the output of the frequency doubler. A frequency doubler of this type does not require any particular balancing circuits in order to maintain the frequency doubling relation, and hence a multiplier of this type is not subject to the disadvantages inherent in frequency dividers and other types of frequency multipliers. Since the frequency supplied by a full wave rectifier or transrectifier is twice the frequency of the fundamental energy supplied to it, all of the octaves are exact. All of the notes of this type of electronic organ may be adjusted by merely adjusting the master oscillators. Such adjustment of the master oscillators does not require any compensating adjustment of the frequency multipliers or apparatus associated therewith.

The vacuum tube 3! contains a pair of diode plates which serve as anodes for a full wave rectifier. These diode plates are connected to opposite terminals of the secondary winding of the transformer 29. In a frequency doubler arrangement such as this, it is essential to eliminate from the output components of the fundamental energizing frequency in order to obtain the proper tonal effect when the frequency doubled energy is supplied as a note in the operation of the instrument. This suppression of the fundamental frequency is obtained by a balancing circuit including a voltage divider 32 and an adjustable capacitor 33. The adjustable capacitor 33 is connected between the adjustable contact arm of the voltage divider 32 and one extremity of the resistance of the voltage divider. The adjustable contact arm is connected through a coupling resistor 34 to the oathode of the vacuum tube 3!. The voltage appearing across the resistor 34 is impressed upon the grid of the vacuum tube 3!.

One of the difiiculties heretofore encountered in the operation of electronic musical instruments, particularly those using frequency dividers, has been an accumulative effect occurring due to a change in the values of the components which eventually result in one or more of the frequency dividers failing to operate. In the correct operation of frequency dividers a rather critical input signal voltage is required. If this input signal is reduced, the output of the divider is also reduced. If it is reduced below a certain minimum the divider fails to divide, and all succeeding dividers in the cascade also fail to operate. This is also accompanied by the failure of such arrangement to provide at substantially constant or comparable levels signals at the different frequencies.

In accordance with the present invention such disadvantages are obviated by supplying a signal to the pentode amplifier tube 3! which overdrives the tube so that the output remains substantially unchanged, regardless of moderate changes in signal input voltage, or tube supply voltages. Another advantage obtained by overdriving the pentode amplifier tube 3| is that the tube will augment the harmonic components supplied thereto from the full wave rectifier input. This method of generation of natural harmonics improves the quality of the tone obtained in an instrument of this type.

The multigrid or pentode vacuum tube 3| is not connected in the conventional manner except that the suppressor grid adjacent the anode is connected to the cathode of the vacuum tube which is grounded. The screen grid interposed between the control grid and the suppressor grid serves as an output anode of the vacuum tube. The screen grid, therefore, is connected through the primary winding of the transformer 35 to an anode potential conductor 36. The transformer 35 is provided with a secondary winding which supplies energy to a succeeding frequency doubler and amplifier stage l3.

The transformer 35 is also provided with an auxiliary secondary winding 31 having one terminal grounded and the other terminal connected to a resistor 38 which is adapted to be connected selectively by operation of a switch 39 to a reproducing amplifier. The operation of the switch 39 produces what is known as a keyed signal which has an attack which is substantially instantaneous. The vacuum tube 3| is also arranged to supply a keyed envelope signal where the attack is relatively slow. This is obtained by connecting the anode of the vacuum tube 3| to an anode resistor 4| having one end connected to a grounded capacitor 42 and a switch 43. The switch 43 is connected to a resistor 44 which is connected to the anode potential conductor 36 and the tuning capacitor 45. The capacitor 45 is connected in parallel with the primary winding of the transformer 35 to tune the primary winding of the transformer 35 to the doubled frequency supplied to the grid of the vacuum tube 3|. The capacitor 45 also operates to increase the voltage available across the secondary winding of the transformer 35 and also acts to suppress harmonics in the transformer.

The anode of the vacuum tube 3| is connected to a coupling capacitor 46 which is adapted to be connected by selective operation of suitable switches or playing keys to the reproducing amplifier. Normally the anode of the vacuum tube 3| does not function since no anode potential is supplied thereto. The closing of the switch 43 supplies anode potential through the attack resistor 44. An appreciable time is required to charge the capacitor 42 through the resistor 44 so that the potential supplied to the anode of the vacuum tube 3| gradually increases. The signal supplied through the coupling capacitor 46 therefore starts with a gradual attack and hence is known as a keyed envelope signal.

When both keys 39 and 43 are actuated together, it is possible to produce an effect very similar to that obtained by certain voicing of a. pipe organ. The keyed signal may be fed through a low pass filter 4? to a reproducing amplifier, and the keyed envelope signal may be fed through a high pass filter 48 to the reproducing amplifier. This will produce an effect corresponding to the flute tone and the reed pipes. The flute pipes have but few harmonics and fairly rapid attack whereas the reed pipes which have a slower attack are usually rich in 6 harmonics. The signal in the secondary winding 31 does not contain appreciable harmonics because of the suppressing action provided by the capacitor 45 across the primary winding of the transformer 35. If the keys or switches 39 and 43 are actuated together, it is possible to produce the effect of reverberation. The rapid attack provided by the keyed signal or the closing of the switch 39 is followed by the slow attack of the key envelope signal due to the closing of the switch 43, which produces a time delay between the two tones which is the effect observed in reverberation.

Another important reverberative effect is obtained when the playing key is released. The keyed signal stops at once. The keyed envelope signal decays slowly, as the charge on capacitor 42 is dissipated through resistor 4| and the plate resistance of tube 3|.

While for the purpose of describing the various tonal effects obtainable from a stage of frequency doubling and amplification indicated within the rectangle l3, mention has been made of the simultaneous closing of switches 39 and 43 and the possible use of filters 47 and 48. It is to be understood that these components and connections are not necessarily included in each of the stages I 3. These various components are actually located outside of the stages and are selectively connected into proper circuit relation by the voicing keys above the organ manuals.

A plurality of frequency doubling and amplification stages are provided in sufficient number for the range of the musical instrument so that each frequency doubling stage is provided foreach successive octave except the last octave. The last octave is produced in the simultaneous frequency doubling and amplifying stage M which has the same components as illustrated in the stage i3 with the exception of the transformer 35:! which is not provided with a main secondary winding for connection to the succeeding stage of frequency doubling and amplification. For simplification in illustration and description the remaining components in the stage |4 have been given reference characters corresponding to those given in the preceding stages such as stage l3.

In Figure 2 there is shown a circuit arrangement employing a master oscillator M, an interstage coupling device I 2 and the succeeding frequency doubler and amplifier stages l3. Interposed between the master oscillator H and the coupling stage I2 is a series resistor 49 arranged between the cathode of the vacuum tube of the master oscillator l and the grid of the coupling device l2. The grid resistor 5| is connected between ground and the grid of the vacuum tube 26 of the coupling device l2. Another oscillator ||a is connected by a coupling resistor 52 to the grid of the vacuum tube 26. The frequencies of the two master oscillators and Ia are adjusted so as to have a frequency difference which is relatively small, of the order of one cycle or less in order to produce a celeste effect. The frequency of oscillator ||a may also be adjusted to be an approximate multiple of the frequency of oscillator II. For this operation it is preferable to have the coupling resistor 52 a great deal larger than the coupling resistor 49. This difference may be such that the resistor 52 has a value from four to ten times the resistance valve of the resistor 49. With such an arrangement signals from the two oscil lators i I and lid are supplied to the frequency doubling and amplifier stages l3 so that there is produced by the reproducing amplifier tones having a slow beat therebetweengenerally referred to as having a celeste or chorus effect. For purposes of illustration only two oscillators II and Ila have been shown, but it is to be understood that still other oscillators may be added so as to emphasize this celestev effect and to produce an effect similar to that obtained by adding a rank of organ pipes in a pipe organ. If the frequency difierence between the two oscillators is of the order of four to siX cycles and the signal supplied by the oscillator Ila is such that it has an amplitude of about one half of the. signal supplied by the oscillator l I to the coupling tube 26, a vibrato effect may be obtained.

While the vibrato efiect may be thus obtained fromthe circuit arrangement shown in Figure 2, it has been found preferable to utilize the circuit arrangement shown in Figure 3. In this figure there is shown a master oscillator lib, which is substantially similar to the master oscillator H master oscillator lib, however, does not have the. lower extremity of the resistor 24 connected directly to the plate potential conductor 2i. The resistor 24 is connected to two series resistors 53 and 54 which are connected to the conductor 2!. The common juncture between the resistors 53 and 54 is connected to a switch 55 adapted to be connected to a variable resistor 56 which has one end grounded. The switch '55 is. adapted to be periodically closed by mechanical means such as a cam 57 driven by a suitable motor 58. This switch 55 preferably opens and closes at a rate of about four to six times per second thus insuring the voltage supplied to the plate or anode of the vacuum tube l to fluctuate at this frequency. In order to smooth these fluctuations so as to make them less abrupt, there is provided a relatively large filter capacitor 5e connected between ground and the common resistors 24, and 53.. This fluctuation of the anode potential of the master oscillator vacuum tube l5 so modifies the feed back to the grid circuit that the resonant frequency of the tuning circuit comprising the inductor it and the capacitor l'i increases and decreases cyclically at a rate corres onding to the frequency of the opening and closing of the switch 55. Thus a frequency difference is produced at a vibrato rate. The resistors 58, 54 and '56 and the switch 55 are common to all of the master oscillators H provided for each half tone. Adjustment of variable resistor 56 permits variation of the degree of vibrato obtained.

The pedal circuit oscillator is illustrated in Figure 4 using a single oscillator vacuum tube 60, of the pentode type. The grid of the vacuum tube '68 is connected to its cathode by a grid resistor BI and the grid is coupled by a grid capacitor 62 to one end of an inductor 63. intermediate point on the inductor 63 is connected to the cathode of the vacuum tube 60. The screen grid and suppressor grid are connected together to a source of anode potential obtained from a conductor 64. A capacitor 65 is connected between these grids and the grounded end of the inductor E3. The anode of the vacuum tube 60 is connected to a conductor 66 which .is connected to a plurality of switches 61 68., 69, and ll. While only four shown in Figures 1 and 2. The 1 switches have been shown, it is to be understood that a greater number actually is provided so that the total number is equal to the total number of half tones provided for the foot pedals. Each of the switches 5'! etc. is arranged to be selectively connected to the correspond-'- ing variable resistors 12, 13, "M, and 15 which correspondin number to the total number of switches. All of these resistors are connected to a fixed resistor l6 which is connected to the anode potential conductor 6 The common juncture between the resistor l6 and the preceding. resistors which were variable is coupled through the capacitor 11 to the grid end of the inductor 63 so as to provide negative feed back. The capacitor ll therefore is connected to a conductor is which is connected to a plurality of switches. i9, 80, 8!, 82 adapted to be connected selectively to a plurality of capacitors 83, 34, 85, and. 86. The switches 79 to 82 correspond in number to the total number of switches provided for the anode circuit, and the total number of capacitors 83 etc. correspond likewise to the total number of switches. Each pedal key is arranged to simultaneously operate two switches at a time. so that for example. a pedal may actuate switches in and 79 to pro duce the note C. Likewise another pedal may be actuated to control switches 68 and to produce C sharp. The capacitors 83 etc. serve to tune the oscillator approximately to the desired frequency, and the adjustable resistors 12 etc. are adjusted to produce accurately each individual tone in a manner corresponding to the adjustment of the master oscillators heretofore described in connection with Figure 1. The foot pedals are also each provided with a switch such as switches 81, 88, 89 and 9| which are interposed between a source of anode potential provided by a conductor s2 and another conductor 93 which is associated with a circuit connected to a keyed envelope signal tube 94. The tube 94 has its grid connected to the common juncture between two resistors 95 and 96. One of the resistors 95 is connected to the cathode of the oscillator vacuum tube 60. The other resistor 96 is connected to ground. The vacuum tube 94 is self-biased by a cathode resistor 91,.

Anode potential is supplied to the vacuum tube 94 through series resistors 98 and 99, the common juncture of which is by-passed by a capacitor Hlfl connected to ground. The anode of the vacuum tube 94 is capacitively coupled by capacitor [0| which feeds to the common amplifier. The circuit arrangement provided for the vacuum tube 94 operates as a keyed envelope signal tube by virtue of the action or" the resistor 98, and the capacitor It!) to provide the requisite slow attack of the pedal notes. The circuit shown in Figure 4 of course may be as- .sociated with various other controls such as the swell or volume control circuit and various tone control or quality control and organ coupler circuits.

In the description of the full wave rectifier and amplifier unit 53 of Figure 1, it was mentioned that it was convenient to use a double diode pentode tube for the dual purpose of a full wave rectifier and amplifier. Where there is no objection to the use of additional tubes, separate full wave rectifier tubes may be used together with the separate amplifier tubes. This is illustrated in Figure 5 where the remaining circuit com ponents have been given reference characters corresponding to those of Figure l. The circuit,

however, uses an individual full wave rectifier tube I02 and an individual pentode tube I03. In this instance the cathode of the rectifier tube I02 may be connected directly to the grid of the amplifier tube I03. A grid to cathode resistor I04 of suitable value is provided to serve the same function as the grid resistor 34 of Figure 1. The cathode of the amplifier tube I03 is provided with a self-biasing resistor 105. The screen grid of the amplifier tube 103 is connected through a coupling resistor I05 to the anode potential conductor 35. The screen grid is also connected through a coupling capacitor ID! to the succeeding stage comprising a frequency doubler and amplifier. Another coupling capacitor I08 is connected between the cathode of the vacuum tube I03 and the succeeding frequency doubler and amplifier stage. While the circuit arrangement shown in Figure 5 does not utilize a trans former having a tuned primary winding, each frequency multiplier stage still utilizes a suppressor network for preventing the fundamental frequency component from appearing at the output of the full wave rectifier. The circuit arrangement in Figure 5 therefore illustrates to some degree the variations and modifications which may be made in the circuit arrangement shown while maintaining the operation provided for in accordance with the present invention.

In each case whether the circuit of Figure 5 or the circuit of Figure l is used the signal volt age supplied to the grid is such as to drive it well beyond cut off so that small changes in the driving voltage will produce no accumulative effect and also produce the desired generation of harmonics by the amplifier tube.

Still another variation of the full wave rectifier amplifier unit 13 of Figure 1 is illustrated in Figure 6 which utilizes a full wave transrectifier circuit. Circuit components corresponding to components used in Figure 1 have been given similar reference characters. The transrectifier circuit utilizes a pair of triodes Ill and H2 having their cathodes connected together and pro vided with a self-biased resistor H3 which is connected to ground. The outer extremities of the voltage divider resistor 32 are connected to the control grids of the triodes HI and H2. While preferably the triodes Ill and H2 should be biased to cut off by a fixed bias in the grid circuit, it has been found satisfactory as shown utilizing the self-biasing resistor 1 E3. The anodes of the triodes H l and H2 are connected together to one terminal of the primary winding of the transformer 85. The output of the transformer 35 may be connected to a succeeding stage hav' ing an input resistor 32. Thus the circuit shown in Figure 6 may be substituted in its entirety for the rectangle 13 in the circuit arrangement of Figure 1. It of course will be appreciated that if desired the circuit of Figure 6 may also be provided with a stage of amplification. Furthermore it will also be apparent to those skilled in the art that the output circuit of the vacuum tubes Hi and H2 may be arranged in a capacitive coupling circuit in a manner similar to that taught by the illustration of the circuit arrangement shown in Figure 5. From the modification shown in Figure 6, it becomes apparent that either a full wave rectifier or a full wave transrectifier may be utilized and the expression full wave rectifier circuit is intended to cover circuits utilizing either the rectifier or the transrectifier. The practice of applicants invention will be readily appreciated by those skilled in the art by consideration of representative values for the various components shown in the figures of the drawings. It may be assumed that in a particular installation it is desired to employ the circuit shown in Figure 1 in a manner so that the oscillator II has a fundamental frequency of 100 cycles. In such instance the inductor l6 would have a value of .5 henry; capacitor I! would be .05 mf.; capacitor 18 .001 mf.; capacitor 22 .2 mf.; and capacitor 25, .005 mf. The resistor l9 would have a value of .5 megohm, resistor 23 would be a voltage divider of .5 megohm, and resistor 24 would have a value of 25,000 ohms.

In the coupling stage I2 the resistor 21 would have a value of 2,000 ohms. In the amplifier unit [3 resistor 32 would be 250,000 ohms, resistor 34, 250.000 ohms, resistor 4|, .5 megohm, resistor 44, 10,000 ohms and resistor 38 .5 megohm. The primary winding on the transformer 35 would have an inductance value of henry. The adjustable capacitor 33 would have a value of 500 micro micro farads. The coupling capacitor 42 would have a value of .1 mf., capacitor 45 would be .012 mf., and capacitor 45 would be .002 mi. Components in the highest octave of Figure 1 would be correspondingly higher depending upon the frequency of the multiplier.

In Figure 2 similar values for the components would obtain and'resistor 49 would be .5 megohm, resistor 51, .1 megohm and resistor 52 of the order of 1 to 5 or more megohms.

In Figure 3 similar values for components would also obtain and resistor 53 would be 10,000 ohms, resistor 54, 15,000 ohms, and resistor 56 of the order of 25 000 to 75.000 ohms.

Typical values for the components in Figure 4 are as follows: capacitor 55, 2 mf., capacitor 02, .02 mf., capacitor 100, .1 mf., capacitor ml, .01 mf., capacitor 11, .05 mf., and capacitors, 83, 84, and 86 approximately .5 mf. depending upon the frequency desired. A typical frequency range of the oscillator of Figure 4 would be about.32 to 64 cycles. The inductor 63 would have a value of about 25 henries.

50,000 ohms, resistor 91, 2,000 ohms, resistors 98 and 99 .5 megohm each, resistor 16, 25,000 ohms, and resistors 12, 73, i4, and 15, .5 megohm voltage dividers. In Figure 6 similar values would obtain with the resistor H3 of the order of 5,000 to 10,000 ohms.

While for the purpose of illustrating and describing the present invention certain particular circuit arrangements and types of vacuum tubes have been shown, it is to be understood that such variations in the circuits and in the components employed therein are contemplated as may be commensurate with the spirit and scope of the invention set forth in the following claims.

This invention is hereby claimed as follows:

1. An oscillator comprising an inductor, a capacitor connected in parallel thereto, a multigrid vacuum tube, means for coupling a portion of said inductor to the control grid and the cathode of said tube, means coupling the remaining grids to the other portion of said inductor, an anode circuit including a plurality of resistors, one of which is variable, and coupling means interconnecting the control grid of said tube with the common juncture between said resistors, one of said resistors having a value substantially equal to the reactance of said coupling means at the operating frequency of said oscillator.

2. The combination comprising'a multi-grid vacuumtube, an input circuit connected to the Resistor 6| would be .25 megohm, resistor 95. 1 megohm, resistor 96,

control grid and the cathode of said tube, an output circuit energized from the screen grid of said tube including a coupling transformer,

said transformer having an output winding for supplying an output signal of a predetermined characteristic, said transformer having a primary winding connected to a source of potential, and circuit means having a time constant arranged for selectively supplying potential to the anode to obtain an output signal therefrom having a different characteristic.

3. The combination comprising a multi-grid vacuum tube, an inputcircuit connected to the control grid and the cathode of said tube, an output transformer connected to be energized from another grid of said tube, said transformer having a plura ity of'secondary windings, one winding being arranged to supply energy to a succeeding frequency doubling stage, the other secondary winding being arranged to supply an output signal of certain characteristics. and circuit means having a time constant arranged for selective y supplying potential to the anode to obtain therefrom an output signal having a different attack.

4. The combination comprising a multi-grid vacuum tube, an input circ it adapted to .be energized from a frequency doubling stage, said circuit being connected to the control grid and the cathode of said tube, an output transformer connected to be energized from another grid of said tube. said transformer having a plurality of secondary windings, one winding being arranged to supply energy to a succeeding frequency doublingstage. the other secondary winding being arranged to supply an output signal of certain characteristics, and circuit means having a time constant arranged for selectively supplying potential to said anode to obtain therefrom an output signal having a different attack, an amplifier, and means for simultaneouslyenergizing said amplifier from said anode circuitand from said output transformer.

, 5. An osci lator comprising a tuned circuit having a capacitor and an inductor, a multi-grid vacuum tube having a control grid connected to one end of said inductor and the cathode connectedto an intermediate point on said inductor, means connectng the other extremityoi said inductor to the remaining grid structure of said tube to serve as the triode anode for saidcircuit, an anode circuit having a plurality of series resistors connected between the anode and the source of anode potential, the resistor adjacent the anode being variable, and capac tive coup'ing means interconnecting the grid end of said inductor with the juncture between said variable resistor and said other resistor. said other resistor having a value substantially equal to the reactance of said coupling means at'the operating frequency of said oscillator. 7

6. In an electronic musical instrument the combination comprising a tuned circuit, a multigrid vacuum tube having the control grid and the cathode connected to said tuned circuit, means connecting another grid of said tube to serve as an anode for said tuned circuit, a circuit connected to the anode of said tube including a resistor and a source of potential, capacitive coupling means interconnecting the grid end of said tuned circuit with a point on said anode resistor, and means for causing the anode potential to fluctuate at a low rate to cause the frequenc of said oscillator to vary at vibrato periodicity. H I

7.'In an electronic musical instrument, the combination comprising a tuned circuit, a multigridvacuum tubehaving the control grid and the cathode connected to a portion of said circuit, means connecting the remainder of said circuit to at least one other grid of said tube to-serve as the'anode therefor, a source of anodepotential, means including a plurality of series connected resistors interconnecting said anode with .said sources of potential, a capacitor interconnecting an intermediate point on said resistors with the grid end of said tuned circuit, and means for varying the potential applied to said anode to produce a fluctuation of the frequency of said oscillator at a rate corresponding to a musical vibrato.

8. In an electronic musical instrument the combination comprising an oscillator adapted to operate over a range of audio frequencies, including an inductor, a multi-grid vacuum tube, means connecting a gr'd and the cathode of said vacuum tube to a portion of'said inductor, means coupling the remaining portion of said inductor to the remaining grid structure ofisaid vacuum tube whereby said grid or grids operates as an anode for said inductor circuit, a plurality of capacitors arranged to be connected to said inductor to form tuned circuits therewith, a plurality of switches arranged for selective operation to connect a selected one of said capacitors to said inductor, a source of potential directly connected to said grid structure serving as an anode, a common resistor connected to a plurality of resistors each of a different value, said common resistor being connected to said source of potential, and a plurality of switches-arranged to selectively interconnect the anode of said vacuum tube with a selected one of said different resstors.

9. In an electronic musical instrument the combination comprising an oscillator adapted to operate over a range of audio frequencies including an inductor, a multi-grid vacuumtube, means connecting a grid and the cathode of said'vacuum tube to a portion of sad inductor, means coupling the remaining portion of said inductor to the remaining grid structure of said vacuum tube whereby said grid or grids operate as an anode for said inductor circuit, a plurality of capacitors arranged to be connected to sa d inductor to form tuned circuits'therewith, a plurality of switches arranged for selective operation, to connect a selected one of said capacitors to said inductor, a source of potential directly connected to said grid structure serving as an anode, a fixed resistor connected to a pluraiity of variable resistors, said fixed resistor being connected to said source of potential, a plurality of switches arranged to selectively interconnect the anode of said vacuum tube with a selected one of said variable resistors, an amplifier, and a coupling means interconnecting said amplifier with said oscillator compr'sing a vacuum tube, a circuit including a plurality of resistors having one end connected to the cathode of said first vacuum tube, means connecting the cathode and the grid of said second vacuum tube across a portion of said resistors, a source of anode potential, an anode circuit for said latter vacuum tube including a plurality of series arranged resistors and a plurality of switches, and means for closing simultaneously one of said switches and one of the switches associated with said capacitors and one of the switches associated with said variable 13 resistors associated with said multi-grid vacuum tube.

10. In an electronic musical instrument the combination comprising an amplifier having a pentode vacuum tube, said tube having a grid, a screen grid, and a suppressor grid, an input circuit connected to the gr'd for driving said grid beyond cut ofi, an output circuit connected to said screen grid, means connecting said suppressor grid to the cathode of said tube, and a second output circuit connected to said anode, a source of potential connected to said screen grid, and means including a switch having a time delay circuit for selectively connecting said source of potential to said anode output circuit.

11. In an electronic musical instrument the combination comprising an amplifier having a pentode vacuum tube, an input circuit connected to the grid of said tube for driving said tube beyond cut off, means connecting the suppressor grid to the cathode of said pentode vacuum tube, a source of potential, an output circuit interconnecting said source of potential and the screen grid of said tube, switch operated means for selectively obtaining from said output circuit a signal to be supplied to a reproducing amplifier, means for coupling the anode of said tube to a reproducing amplifier, means including a switch and a time delay circuit for selectively connecting said source of potential to said anode whereby an increasing anode potential is applied to cause an increasing signal to be supplied to a reproducing amplifier, and means including said switches for simultaneously supplying to the reproducing amplifier signals from said anode and said screen grid output circuits for producing a reverberation effect.

12. In an electronic musical instrument the combination comprising a reproducing amplifier, an amplifier adapted to supply signals to said first amplifier, said amplifier having a pentode vacuum tube energized from a source of audio frequency signal for driving said tube beyond cut ofi, means connecting the suppressor grid of said pentode tube to the cathode thereof, a source of anode potential, an output circuit including a transformer having a primary winding connected between said source of potential and the screen grid of said tube, said transformer having a secondary winding, a switch interconnecting said secondary winding with said reproducing amplifier, coupling means interconnecting the anode of said tube with said reproducing amplifier, an anode coupling resistor, a resistance capacitance time delay circuit, switch means for connecting said time delay circuit and said anode coupling resistor to said source of potential.

13. In an electronic musical instrument, the combination comprising a tuned circuit, a multigrid vacuum tube having the control grid and the cathode connected to a portion of said circuit, means connecting the remainder of said circuit to at least one other grid of said tube to serve as an anode therefor, a circuit for the anode of said tube, variable means interconnecting said anode circuit with said tuned circuit to provide feed back, and means for varying the amount of feed back between certain limits at a periodic rate to produce a fluctuation of the frequency of said oscillator at a rate corresponding to a musical vibrato.

14. In an electronic musical instrument the combination comprising an amplifier having a multi-grid vacuum tube, an input circuit connected to the grid of said tube for driving said grid beyond out off, an output circuit connected to another grid of said vacuum tube, and a second output circuit connected to the anode of said vacuum tube, a source of anode potential, and means including a switch having a time delay circuit for selectively connecting a source of anode potential to said second output circuit,

THOMAS J. GEORGE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 20,293 Dow Mar, 16, 1937 736,884 Shoemaker et a1. Aug. 18, 1903 1,765,606 Ohl June 24, 1930 1,848,507 Vos et a1 Mar. 8, 1932 1,885,728 Keith Nov. 1, 1932 2,161,706 Hammond June 6, 1939 2,169,762 Kaye Aug. 15, 1939 2,195,853 Fitch Apr. 2, 1940 2,221,188 Hammond et al. Nov. 12, 1940 2,229,755 Manatt Jan. 28, 1941 2,231,687 Shepard, Jr Feb. 11, 1941 2,251,025 Albricht July 29, 1941 2,276,389 Hammond Mar. 17, 1942 2,276,390 Hanert Mar. 17, 1942 2,340,002 McKellip et a1 Jan. 25, 1944 2,357,191 Hanert Aug. 29, 1944 

